Miniature variable resistance device



March 7, 1961 K. w. YOUNGBECK EFAL 2,974,299

MINIATURE VARIABLE RESISTANCE DEVICE E Y R HMM 1 \lu a 3 m m m e 9 EOH Twme 0.6 A M WRK w l s w w R A. R A N A KCD.

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March 7, 1961 K. w. YOUNGBECK ETAL 2,974,299

MINIATURE VARIABLE RESISTANCEDEVICE Filed Jan. 2, 1959 3 Sheets-Sheet 2o N 3 r?) :2 4) N L Q N 2 N N N m r I u. \n N 3 1 El 00 g, o m m m d)INVENTOR. KARL W. Youuesecx CHARLES R. Gone DONALD K. SCHUETTE.

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March 7, 1961 K. w. YOUNGBECK ErAL 2,974,299

MINIATURE VARIABLE RESISTANCE DEVICE Filed Jan. 2,1959 3 Sheets-Sheet 31,111,111,. 'IllIIIIIIIIIIIIIIIIII'I'"IIIII'IIIIIIIII o (.L m i;

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KARL W. YouNe-eEcK CHARLES R Goaae,

DONALD K. ScHuETTE 5 d /4 4! MM ATTORNEY 'minals molded in the base.

nited S te a t MINIATURE VARIABLE RESISTANCE DEVICE Karl W. Youngbeck,Charles R. Goerg, and Donald K. 'Schuette, Milwaukee, Wis., assignors toGlobe-Union Inc., Milwaukee, Wis., a corporation of Delaware Filed Jan.2, 1959, Ser. No. 784,561

Claims. (Cl. 338--163) This invention relates to improvements invariable electrical resistance devices, especially those of the typeused with miniature electrical apparatus such as hearing -is smallerthan those heretofore made and may be used for small transistor typehearing aids and other miniature applications.

Another object is to produce a highly compact, sensitive variableresistance device which is inexpensive to manufacture.

The first of these objects is accomplished by a novel arrangement ofparts which reduces the size of the present device to a compactnesspreviously unobtainable.

In order to conserve thickness, a thin, flat-sided, molded 'base isemployed. All the flat terminals are molded into the base and projectfrom an edge between the fiat sides. The shaft for mounting the rotatingelement is also 'molded into the base and does not project beyond the Itis connected outer or mounting side of the base. within the base to thecenter terminal. To further conserve thickness and also maintainsensitivity the inside of the base has an annular groove in which isflushly seated and cemented a preformed resistance element. Suchresistance element when made as a separate unit can be held to closeelectrical tolerances anduniformity. The electrical connection betweenthe ends of the resistor element and the outer terminals is accomplishedby silver painting the ends of the resistance element and employingsilver epoxy cement to secure the silvered ends of the resistanceelement to exposed surfaces of the ter- In order to further eliminateunnecessary thickness and still retain sensitivity and freedom fromoperating noise the rotating element consists of a cover-knob whichcompletely encloses the electrical elements and rides against theperiphery of the base to effect a seal and prevent wobble. The rotatingelements consist of a contact spring and a center take-off spring which,with the cover, are staked to a bushing. The bushing is rotativelymounted on the shaft by a retaining ring on the end of the shaftprojecting through the bushing. Good electrical connection isestablished by the fixed engagement between the two springs as staked'to the bushing and the spring biased wiping connection between thecenter take-off spring and a shoulder on the shaft. This eliminates theuneven and unsatisfactory electrical connection between the bushing andthe shaft.

Conservation of diameter is obtained primarily by the use of a preformedresistance element which can provide the proper resistance with theshortest circumferential length.

2,974,299 Patented Mar. 7, 1961 Considered without the locating nibs onthe mounting side of the base the overall thickness of an efiicientvariable resistor made in accordance with this invention may be lessthan 0.115 inch and its diameter less than 0.283 inch.

The second object is attained by use of a minimum number of small partswhich are held in subassemblies during the molding operation tofacilitate handling.

Fig. 1 is an enlarged top view of a variable resistor embodying thisinvention;

Fig. 2 is an enlarged sectional view taken on the line 22 of Fig. 1showing the contact spring in contact with the resistance element;

Fig. 3 is an enlarged sectional view taken on the line 3--3 of Fig. 1;

Fig. 4 is an enlarged bottom view of the movable element removed fromthe unit;

Fig. 5 is an enlarged top view of the base with the movable elementremoved showing the contacts, resistance element and mounting shaft;

Fig. 6 is an enlarged fragmentary sectional view taken on the line 66 ofFig. 5;

Fig. 7 is a top view of the base showing the terminals and mountingshaft molded in place before detachment of the terminal strip;

Fig. 8 is a sectional view taken on line 8-8 of Fig. 7; and

Fig. 9 is a bottom view of the base shown in Fig. 7.

The variable resistor exemplifying one form of this invention has a base1 molded of any suitable nonconductive material such asphenol-formaldehyde or polyester molding compounds. A shaft or post 2and terminals 4, 5 and 6 are molded into the base and are relativelypositioned as shown in Fig. 5. As can be seen best in Fig. 9, thecenter-terminal 4 has a collar 34 in which the shaft is staked to assurea good electrical contact and provide a seat for anchoring the assemblyin and strengthening the base. The terminals 5 and 6 are molded into thebase so that their inner ends, 9 and 10 respectively, underlie and areexposed in an annular groove 7 into which a resistance element 8 isflushly seated. The resistance element consists of a blanked outlaminated phenolic strip with resistance paint on its upper surface.Such preformed elements are well known to those skilled in this art andcan be made to close electrical tolerances and uniformity. The ends ofthe resistance element are coated with silver paint to provide a goodelectrical path from, the bottom of the element at such ends to theohmic path on its top. When the element is cemented in the groove 7,silver laden epoxy is placed between the ends 9 and 10 and the silveredends of such element to establish a good electrical connection betweenthe terminals 5 and 6 and the resistance element 8 as shown in Fig. 6.The arrangement of flat terminals molded in the thin, flat-sided baseand projecting from one edge of such base permits the thickness of thebase to be kept at a minimum. In

some installations the flat bottom of the base is cemented to thechassis of the hearing aid or the like. In other installations it isadvisable to have locating nibs 39 projecting from the base toaccurately determine the location of the unit.

The method of molding the base greatly facilitates the speed of itsmanufacture and the accurate positioning of the terminals 5 and 6 withrespect to the resistance element 8. Figs. 7 and 9 best show thismethod. A terminal carrier strip 32 with spaced openings 33 has groupsof properly spaced terminals 4, 5 and 6 blanked out as integral parts.Oblong ends 35 on the shafts 2 are fitted in openings in the collars 34of terminal 4 and parts thereof staked over such collars as indicated at36. A multiple cavity casting die and an associated jig have externalpins which fit in the spaced holes 33 to properly align the terminalgroups with respect to each cavity. The inner ends of the terminals and6 and oblong end 35 of the shaft rest on supports in the cavities toproperly center them within the base when molded. These account for thedepressions 37 and 38 (see Fig. 9). The die is closed and moldingmaterial forced into the cavities in a well known manner. As the jig anddie are well known in the art they are not shown. The important item isthe use of a carrier strip onto which the bases are molded and by whichthey are carried for further assembly operations. Such operations mayinclude the insertion and cementing of the resistance element 8 in thegroove 7 and assembly of the moving elements hereinafter described. Whenassembly operations are completed the units are disconnected from thecarrier strip 32 by severing the terminals at the desired places.

The moving elements mounted on the shaft 2 consist of a cover oroperating knob '22, a contact spring 16 and a take-off spring 24 allfixedly secured by a headed bushing 11. The bushing 11 has a centralbore which rotatably fits freely on the shaft 2. At its lower end thebushing has a downwardly extending projection 14 (see Figs. 2 and 4)which engages with a stop 15 formed on the shaft 2 (see Figs. 3 and 5)to limit the rotation of the movable elements with respect to theresistance element 8. The body of the bushing has opposed slots 12 (seeFig. 3) which receive tabs on the cover, contact spring and take-offspring to prevent relative rotation.

The bushing is fitted into an opening in the cover 22 with its headedend out. Cover tabs 23 extending into such opening fit in the slots 12.The cover 22 has a peripheral flange 31 which makes an enclosure for theresistance element, movable contact spring and take-oif spring. The endof such flange rides on the inner face of the base to form a dust sealand a broad bearing preventing wobble of the moving elements withrespect to the shaft.

The contact spring 16 is substantially circular in shape. It has aC-shaped slot 17 which divides it into an outer ring 18 and inner disc19. A nipple 21 formed on the outer ring 18 rides on the resistanceelement 8. A central opening in the disc 19 has a tight fit on thebushing 11 and tabs 20 on the edge of such opening fit snugly in theslots 12. The inner disc 19 seats snugly against the inner wall of thecover 22 and the outer ring 18 is biased out from the plane of such discas shown to keep a spring pressed sliding engagement between nipple 21and the resistance element 8 to provide a substantially noiselesselectrical contact during variance in the electrical resistance of thedevice. 7

While an electrical contact is established between the contact spring 16and the center terminal 4 through the bushing 11 and shaft 2, suchcontact is not satisfactory because of variation in fit and pressuresduring operation. Consequently, the shaft 2 is provided with an annularshoulder or face 3 flush with the inner side of the base 1 against whichthe take-off spring 24 has a spring biased sliding contact not variableduring operation. The takeoff spring 24 has a C-shaped portion 25 (seeFig. 4) and a substantially circular portion 26 joined by a short U-shaped bridge 28 (see Fig. 2). An opening in the portion 26 fits snuglyover the bushing 11 and tabs 27 on such portion fit into the slots 12.The portion 26 rests flatly against the inner disc 19 and parts 13 (seeFig. 4)

of the metal of bushing 11 adjacent the slots 12 are staked over theportion 26 to tightly hold the cover and both springs in the assembledposition. The C-shaped portion 25 has its ends provided with dimples 29and such portion is biased outwardly from the portion 26 to maintain asliding electrical contact between such dimples and the shoulder 3.

The movable cover and contact spring assembly is rotatably held on theshaft 2 by a retaining ring 30 which is snapped into a groove near theouter end of shaft 2.

The tolerances are such that the wall of the cover is slightly deformedto maintain a sliding bearing between the edge of the flange 31 and thebase 1. This is important not only to establish a seal against dust andlint but also to hold the cover 22 in selected position. In additionsuch bearing prevents the cover from wobbling when rotated thus insuringeven contact between the sliding electrical connections.

The operation of the variable resistor is in accordance with well knownprocedure. The cover is rotated within the permissible limits to varythe resistance. The problem solved by this invention is the reduction ofsize while maintaining necessary range, uniformity, long life,prevention of objectionable noise, and freedom from interference causedby dirt and lint.

We claim:

1. In a variable resistance device, the combination comprising, a moldedbase of insulating material having flat outer and inner sides, saidinner side having a groove, a conductive shaft fixedly mounted on saidbase, and projecting from said inner side only, a resistance elementsecured in said groove, a plurality of terminals molded in the base, atleast one of said terminals being in contact with said shaft and otherof said terminals contacting said resistance element, a bushingrotatably mounted on said shaft, a cover secured to said bushing andhaving a downturned peripheral flange slidably bearing on said moldedbase, a contact spring secured to said bushing within the confines ofsaid cover and said flange and biased into sliding contact with saidresistance element, a takeoff spring secured to said bushing and to saidcontact spring within the confines of said cover and said flange andbiased into sliding electrical contact with said shaft, and a retainingring on said shaft to hold said bushing in place thereon.

2. In a variable resistance device, the combination comprising, a thinfiat-sided molded base, a groove in one side of said base, a conductiveshaft molded integrally into said base and projecting only from said oneside, a resistance element in said groove, fiat terminals moldedintegrally into said base and projecting from an edge between saidsides, one of said terminals being in electrical contact with saidshaft, others of said terminals being in electrical contact with saidresistance element, a movable assembly including a cover and a resistance element contacting spring rotatably mounted on said shaft, saidcover having a sliding bearing on said base, and means reacting betweensaid shaft and said cover to keep said cover in sliding bearing withsaid base.

3. A miniaturized sensitive variable resistance device comprising, aflat-sided thin base having a peripheral flat bearing surface, apreformed resistance element in said base flush with one side thereof, ashaft fixed in said base and projecting from said one side, said shafthaving a contact face flush with said one side, terminals connected tosaid resistance element and said shaft, a cover having a flange in flatbearing contact with said bearing surface, said cover being rotatablyguided by said shaft, spring biased contacts carried with said cover andengaging said resistance element and said contact face, and means actingbetween said shaft and said flange to maintain it in sealing bearingengagement with said bearing surface.

4. The resistance device as claimed in claim 3 in which said meansincludes the inherent flexibility of said cover.

5. In a miniaturized variable resistance device, a movable housing, asubstantially circular contact spring having a concentric C-shaped slotdefining an inner disc with a central opening and an outer ringresiliently flexed out of the plane of said disc, a resistance elementengageable nipple carried on said outer ring, a bushing in said openingfixedly mounting said disc to said housing, and a take-off, springhaving a substantially circular portion with a central aperturesurrounding said bushing and a O-shaped portion having take-off faceengageable contacts on the extremities of said C-shaped portion andjoined to said circular portion by a short bridge, said C-shaped portionbeing resiliently flexed at an angle with respect to said circularportion, said circular portion being 5 fixedly mounted on said innerdisc by said bushing.

References Cited in the file of this patent UNITED STATES PATENTSBenkelman Oct. 17, 1939 Shapiro Sept. 21, 1943 Chanowitz Aug. 23, 1949Richman et a1. Nov. 20, 1951

